In general, an OLED display is a display device that electrically excites fluorescent organic material for emitting light and displays an image by voltage programming or current programming N×M organic light emitting cells. An organic light emitting cell of the OLED display includes an anode (which may be made from indium tin oxide, or ITO), an organic thin film, and a cathode layer (which may be metal). The organic thin film has a multi-layer structure including an emitting layer (herein referred to as EML), an electron transport layer (herein referred to as ETL), and a hole transport layer (herein referred to as HTL) so as to balance electrons and holes to thereby enhance light emitting efficiency. Further, the organic thin film separately includes an electron injection layer (herein referred to as EIL) and a hole injection layer (herein referred to as HIL).
Methods of driving the organic light emitting cells having the foregoing configuration include a passive matrix method and an active matrix method employing a thin film transistor (herein referred to as TFT) or a metal-oxide semiconductor field-effect transmitter (herein referred to as MOSFET). In the passive matrix method, an anode and a cathode are formed crossing each other, and a line is selected to drive the organic light emitting cells. In the active matrix method, an indium tin oxide (herein referred to as ITO) pixel electrode is coupled to the TFT, and the light emitting cell is driven by a voltage maintained by capacitance of a capacitor. Herein, the active matrix method can be classified as a voltage programming method or a current programming method depending on the type of signals transmitted to the capacitor to distinctively control the voltage applied to the capacitor.
A pixel circuit according to a conventional voltage programming method has difficulties in expressing high-level gray scales due to deviations of threshold voltages VTHs of TFTs and/or mobilities of carriers of the TFTs, the deviations being generated as a result of a non-uniform manufacturing process of the TFTs. On the other hand, although currents and/or voltages supplied driving transistors in a plurality of pixel circuits may not be uniform, a pixel circuit employing a current programming method can provide panel uniformity as long as a current supplied from a current source to the pixel circuits is uniform.
When utilizing a display device by using the pixel circuit that employs the current programming method, a grayscale current generating circuit is required to convert grayscale data into a grayscale current to apply the grayscale current to the pixel circuit.
The above information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and therefore, unless explicitly described to the contrary, it should not be taken as an acknowledgment or any form of suggestion that this information forms the prior art that is already known in this country to a person skilled in the art.